Extensive studies on the third variable region, V3, of HIV-1 provided evidence that variation in V3 is involved in determining the antigenicity and cell specificity of HIV-1. Gp120 of SlV contains regions corresponding to the V3 of HIV-1; however, little is known about the biological and genetic properties of SIV V3. The SIV V3 does not appear to be variable in peripheral blood leukocytes (PBL) of persistently infected macaques as well as among laboratory isolates. However, we have observed considerable genetic variation in V3 in tissues of rhesus macaques that died from AIDS following infection with cloned SIVmac239. The pattern of amino acid variation in SIV V3 was not random, similar to those in HIV-1 V3. The amino acids comprising the predicted crown and stem regions of the V3 loop were well conserved; however, the regions flanking the crown on both sides were highly variable. Molecularly cloned SIV containing one of the naturally mutated V3 sequences in gp120 showed higher replicative and cytopathic ability for T-lymphocytes than SIVmac239. The mutated V3 sequences contain 6 amino acid changes within the 32 amino acid stretch of the V3. The mutated V3 is more positively charged and hydrophilic than V3 of SIVmac239, which are properties similar to V3 of HlV-1 with high replicative and cytopathic ability for T-lymphocytes. These observations strongly suggest that SIV V3 might be subjected to similar selective pressures that occur on HIV-1 V3, and indicate that V3 in SIV plays a role in the pathogenesis of SIV infections, as it does in HIV-1. In these studies, we propose to investigate the biological significance of SIV V3 for viral cell tropism and antigenicity by using naturally mutated V3 sequences. The V3 of T-cell tropic and macrophage tropic cloned SIV will be exchanged with the naturally mutated V3 sequences, and V3 recombinant viruses will be generated. The significance of the V3 sequences for cell-tropism, virus entry, and immunogenicity will be biochemically analyzed. These studies will shed new light on the biological significance of SlV V3 and will provide useful information for vaccine studies using the SIV model system for human AIDS.